The final fight is just that I think....final.
You can either launch HoW importing your characters (that are saved before the final IWD battle, not after, if you have HoW installed) or don't make the final IWD battle, go to HoW trough Kuldalar door, end HoW and go back to the final IWD battle (I'm not sure about this second hypothesis, but it should be so).
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Wizbane